Communication: Mode-dependent excited-state lifetime of phenol under the S₁/S₂ conical intersection

Phenol can serve as a model for examining the deactivation of the aromatic amino acid tyrosine following UV excitation, which mainly occurs through a repulsive πσ^∗ state along the O-H bond. The reaction barrier formed by the conical intersection between the optically bright S₁ (ππ^∗) state and the dissociative S₂ (πσ^∗) state does not inhibit O-H bond rupture even though the excitation energy is below the barrier height. To examine the O-H bond-rupture dynamics in association with the initial excited vibrational modes, we used a picosecond laser to investigate the vibrational-mode-dependent excited-state lifetime of phenol under the S₁/S₂ conical intersection. Unexpectedly short lifetimes were observed in the S₁ state for a″ symmetric vibrational modes (including v₄, v_16a, τOH, and v₅). These results clarify recent theoretical calculations showing that the relaxation from S₁ to S₂ either occurs via symmetry-allowed non-adiabatic transitions or is topographically linked to a lower energy minimum on the multidimensional potential energy surface.